A rotor of an induction motor is configured of an electrical sheet, a rotor bar, and end-ring. In the case of a typical induction machine, since the rotor bar and the end-ring are made of an aluminum material, the rotor bar and the end-ring are simultaneously manufactured to integrate the rotor bar and the end-ring by aluminum die casting. Recently, in order to improve efficiency of an induction motor, it is inclined to use copper having low resistivity instead of using aluminum.
However, in the case of using copper instead of aluminum, it is very difficult to manufacture the induction motor by copper (Cu) die casting. The reason is that a boiling point of aluminum is 700° C. but a boiling point of copper is 1300° C. In the case of manufacturing the induction motor by the copper die casting, an expensive die casting equipment is required and since a manufacturing technology of only the copper die casting is required, a domestic/foreign place which may manufacture the induction motor is very restricted.
For this reason, the copper rotor bar has been typically used. In this case, after the copper rotor bar fitted for a shape of the electrical sheet is manufactured and is inserted into the electrical sheet, upper and lower copper rotor bars and the end-ring portion are coupled with each other by brazing.